I. Field of the Invention
The present invention relates generally to navigation systems and, more particularly, to a navigation system with means for geographic interpolation of traffic data.
II. Description of Related Art
Navigation systems of the type used in automotive vehicles have enjoyed increased popularity in recent years. Such navigation systems are particularly useful for providing route information on a display screen visible to the operator of the motor vehicle. In order to obtain the directions to a desired destination, the operator of the motor vehicle typically inputs the address of the destination by a touch screen or other means. After the destination has been inputted to the navigation system, it then computes the desired route which is typically the fastest route between the current location of the vehicle and the destination.
In order for the navigation system to compute the route from the current location of the vehicle and to the destination, these previous navigation systems have included a map database which includes map data used in the route calculations. Such map data typically comprises a plurality of meshes wherein each mesh is assigned a predetermined geographic area. The map data in each mesh includes not only the position, length, speed limit, etc. for each road link, but also the type of road, e.g. interstate highway, two lane road, etc., for each road link.
In order to achieve the highest degree of accuracy, e.g. minimum travel time, in the route calculations to the destination, ideally traffic flow data, e.g. real-time data or statistical data, is available for each road link in each mesh. Such statistical data may be obtained by the navigation system in any of numerous known fashions.
Unfortunately, traffic flow data for many geographic areas is unavailable. Consequently, navigation systems have utilized the speed limit for the various road links in the meshes which do not contain traffic flow data when performing route calculations.
Such calculations of route data based upon the speed limit in meshes which do not contain traffic flow data but in which the speed limit is defined, however, have proven somewhat inaccurate in operation. In particular, the vehicle speed on any given road link is typically, but not always, less than the posted speed limit for the road link. Consequently, since the previously known navigation systems statically preassign a speed value derived from the speed limit for road links of the same type in meshes which do not contain traffic flow data when performing route calculations, errors may occur resulting in inaccurate routing of the optimal route between the location of the vehicle and the destination.